LONDON—After a summer in which stocks have hit a seemingly never-ending run of record highs, September brings a series of monetary and political events that could jolt investors out of their complacency. The will-they-won’t-they debate over trimming pandemic-era stimulus gets an airing with several G10 central banks holding meetings. A showdown over U.S. national debt alongside crucial elections in Japan and Germany add to the risks. Here are eight events, listed chronologically, which investors will watch in September: Taper Down Under—Sept. 7 The Reserve Bank of Australia meets on Tuesday and provides September’s first test of central banks’ determination to stick with plans to cut stimulus. Last month, it stuck with plans to reduced weekly bond buying to A$4 billion ($2.9 billion) in September from the previous A$5 billion. Yet the rapid spread of the Delta variant and a slowing economy are piling pressure on the RBA to delay the …